1. Field of the Invention
The present invention generally relates to ground fault protection, and more particularly to an AC ground fault sensor system for detecting ground fault conditions.
2. Description of the Prior Art
The high voltage requirements of traction batteries used to power either electric vehicles or hybrid electric vehicles (HEV) raise significant electrical safety concerns. Chief among these safety concerns are ground fault conditions. A ground fault is an unwanted electric current flow outside of the intended electric circuit flow which can cause significant damage to electronic components within a system (such as an HEV propulsion system), thereby disabling or even destroying the electronic equipment.
Therefore to reduce the likelihood of a shock many conventional traction battery systems employed by electric vehicle or HEV do not have one terminal grounded to the automobile chassis, unlike the typical low voltage automotive storage batteries employed by internal combustion engine vehicles.
FIG. 1 depicts a schematic illustration a conventional power stage 100 of a DC/AC converter, the power stage 100 includes a first power conductor (positive rail) for providing a positive DC link 110 (hereafter “+DC link”) and a second power conductor (negative rail) for providing a negative DC link 120 (−DC link”). A power source such as a string of batteries (not shown) is typically coupled to a positive terminal 112 of the +DC link 110 and a negative terminal 122 of the −DC link 120 for providing a high voltage (e.g. 600 volts) power source to drive an electric traction motor (not shown). A conventional balance circuit 130 and a switching mechanism 140, as more fully described below are also provided in the conventional power stage 100. Moreover, DC bus capacitor C3 is coupled to the +DC link 110 and the −DC link 120 for providing stabilization for maintaining the voltage difference (DC Link) between the +DC link 110 and −DC link 120.
As mentioned above, the conventional power stage 100 of a DC/AC converter includes a switching mechanism 140 couple between the +DC link 110 and −DC link 120 including three pairs of switches sw1: Q1D1: Q2D2, sw2: Q3D3: Q4D4, and sw3 Q5D5: Q6D6. Each switching pair includes two NPN transistors and two diodes coupled to common terminal A, B and C respectively. For example, in switch number one sw1, NPN transistor Q1 collector is couple to first power conductor 110 and Q1 emitter is coupled to the anode of first diode D1 and the cathode of second diode D2 and common terminal A. Second NPN transistor Q2 emitter is coupled to the second power conductor 120 and Q2 collector is coupled to Q1 emitter as well as D1, D2 and common terminal A. Common terminals A, B and C are respectively coupled to each phase lead of a multi-phase motor (not shown). Moreover, the switches, sw1, sw2, and sw3 open and close according to a predetermined switching rate to selectively couple power to a given phase through either the positive or negative power conductor links. The switches can be pulse width modulated (PWM) to drive various motor phases and create an AC waveform as known to those skilled in the art. Moreover, the switches may for example take the form of a variety of semiconductor switching devices including field-effect transistors FETs, insulated-gate bipolar transistors IGBTs, bipolar transistors, and silicon controlled rectifiers SCRs as known to those skilled in the art.
In addition to the switching mechanism 140, the conventional power stage 100 of a DC/AC converter includes a conventional balancing circuit 130 which includes a first balance resistor R1 connected to the first power conductor 110 and in series with a second balance resistor R2 where R2 is coupled to the second power conductor 120. Disposed between R1 and R2 is node N2 which is also coupled to a ground (hereafter “vehicle chassis ground” 132). By providing first and second balance resistors R1 and R2, the conventional power stage 100 of a DC/AC converter attempts to provide equally apportioned voltage potentials at the vehicle chassis 132 with respect to +DC link 110 and −DC link 120. In other words, voltage potential at the voltage chassis ground 132 with respect to the −DC link 120 and from +DC link 110 with respect to the vehicle chassis ground 132 are both equal to half of the DC link voltage Vdc. However, it should be noted that power electronics system utilizes switching technology to synthesize waveforms, such as by means of switching mechanism 140 result in swings in the voltage potential from vehicle chassis ground 132 to −DC link 120 due to the coupled leakage capacitance through phase output leads from a electric traction motor. This vehicle chassis voltage swing can generate noise in a low voltage control system, which ultimately can cause system failures.
To alleviate vehicle chassis voltage swing, conventional balancing circuit 130 includes snubber capacitors, C1 and C2 coupled to node N1, node N2 and vehicle chassis ground 132. The snubber capacitors, C1 and C2 attempt to bypass vehicle chassis voltage swing to achieve stable vehicle chassis voltage. In essence, snubber capacitors, C1 and C2 are provided to reduce or eliminate the AC chassis voltage that appears between vehicle chassis ground 132 and either DC link 110 or DC link 120. For example, if you run a conventional DC/AC converter without grounded snubber capacitors, C1 and C2, the parasitic capacitance from the three phases (A, B, and C) of an electric traction motor (e.g. from the motor windings and cables) would be the lowest impedance in the system resulting in the average A, B, and C phases being imposed between vehicle chassis ground 132 and either +DC link 110 or the −DC link 120. Therefore, conventional balancing circuit 130 attempts to provide an isolating system which minimizes the likelihood of a significant electric shock to electronic equipment in the event of a short circuit or low impedance connection between a phase lead of a motor and the vehicle chassis ground (e.g. a ground fault).
As shown in FIG. 1, snubber capacitors, C1 and C2 are connected via node N1 which is also coupled to node N2. During normal operating conditions, the snubber capacitors and the leakage capacitance provide a path for current to flow at multiple switching frequencies of a power electronics device. Hence, the magnitude of this current is limited by the leakage capacitance. However, as discussed above, when the phase winding is shorted to the vehicle chassis ground 132, a high magnitude fault current is generated which can cause damage to products and equipment. Accordingly, to avoid damage to products and equipment (e.g. the DC/AC converter itself or other products and equipment coupled to the DC/AC converter) snubber capacitors C1 and C2 are typically high capacitance capacitors, which can attempt to continuously sustain a high magnitude of fault current. However, once the magnitude of the fault current exceeds the protective capability of snubber capacitors C1 and C2 products and equipment are susceptible to damage.
Hence, it would be desirable to provide an AC chassis fault detection system, which would indicate a ground fault to protect products and equipment before snubber capacitors C1 and C2 fail to protect the system. Moreover, it would be desirable to have an AC chassis fault detection system, which would be robust in that the detection system could determine if a short exists due to a wide-range of fault conditions. That is, it would be desirable to have an AC fault detection system which could detect an AC fault regardless of the type of electric traction motor attached to an DC/AC converter or the length of the cables connecting each phase (A, B, or C) of the electric traction motor.